GW170817 Most Likely Made a Black Hole

Abstract: There are two outstanding issues regarding the neutron-star merger event GW170817: the nature of the compact remnant and the interstellar shock. The mass of the remnant of GW170817, $\sim$2.7 $M_\odot$, implies the remnant could be either a massive, rotating, neutron star, or a black hole. We report Chandra Director's Discretionary Time observations made in 2017 December and 2018 January, and we reanalyze earlier observations from 2017 August and 2017 September, in order to address these unresolved issues. We estimate the X-ray flux from a neutron star remnant and compare that to the measured X-ray flux. If we assume that the spin-down luminosity of any putative neutron star is converted to pulsar wind nebula X-ray emission in the 0.5-8 keV band with an efficiency of $10^{-3}$, for a dipole magnetic field with $3 \times 10^{11}$ G < $B$ < $10^{14}$ G, a rising X-ray signal would result and would be brighter than that observed by day 107, we therefore conclude that the remnant of GW170817 is most likely a black hole. Independent of any assumptions of X-ray efficiency, however, if the remnant is a rapidly-rotating, magnetized, neutron star, the total energy in the external shock should rise by a factor $\sim$$10^2$ (to $\sim$$10^{52}$ erg) after a few years, therefore, Chandra observations over the next year or two that do not show substantial brightening will rule out such a remnant. The same observations can distinguish between two different models for the relativistic outflow, either an angular or radially varying structure.

• The paper shows that the observed X-ray emissions are too weak for a neutron star, supporting the black hole remnant scenario.
• It uses multi-epoch Chandra data to compare expected pulsar wind nebula brightness with measurements, challenging prior neutron star models.
• The study also examines jet structure implications, illustrating how external shock dynamics can refine our understanding of merger outcomes.

Analysis of the Neutron-Star Merger Event GW170817 and Its Implications

The paper "GW170817 Most Likely Made a Black Hole" by Pooley et al. explores the enigmatic aftermath of the neutron-star merger event GW170817, specifically examining the identity of the compact remnant formed. The paper provides a detailed account of X-ray observations using the Chandra Director's Discretionary Time, employing these data to ascertain the remnant's nature while exploring the dynamics of the interstellar shock induced by the merger.

Summary of Observations and Analysis

The authors conducted Chandra X-ray observations in several epochs following the merger event and reanalyzed earlier data from the initial detection period. These observations aimed to clarify the characteristics of the remnant and the relativistic outflow structure. The theoretical foundation rests on the established understanding that the remnant, with its estimated mass of approximately 2.7 solar masses, could be either a massive neutron star or a black hole.

For a neutron star remnant with rotating dipole magnetic fields of $3\times10^{11}$ G $< B < 10^{14}$ G, an insightful analysis was conducted to compare the expected X-ray emissions against those observed. If indeed a neutron star were present, the spin-down luminosity, when transformed into pulsar wind nebula X-rays, should significantly surpass the observed X-ray flux by day 107 post-merger with an efficiency of $10^{-3}$. However, the data contradicted this scenario, leading to the conclusion that the remnant is more likely a black hole due to the insufficiency of observed X-ray brightening.

Predictions and Further Implications

Chandra's observations elucidate possibilities for distinguishing between different jet models — whether the relativistic outflow is defined by radial or angular variations. These model implications stand pivotal in understanding jet structures in high-energy astrophysical phenomena, such as those associated with kilonovae and gamma-ray bursts derived from neutron-star mergers.

In the broader astrophysical context, the paper suggests that the lack of significant X-ray flux increases also challenges the hypothesis of a magnetized, rapidly rotating neutron star. By observing potential shifts in the external shock's energy, future observations could further validate these conclusions. An angularly structured jet would be steadfastly characterized by energy that decreases with angle from the jet axis — an off-axis observer, as per the analysis, would consistently observe an increasing light curve until the deceleration mitigates beaming effects.

Future Developments and Considerations

If Chandra observations in subsequent years do not show substantive brightening, they will robustly confirm the absence of a neutron star remnant, thereby reinforcing the candidate of a black hole as the aftermath of GW170817. The findings also signal an imperative need for continued longitudinal studies using multi-wavelength observational strategies to disentangle the complex interaction dynamics post-merger events.

As gravitational wave detection and electromagnetic follow-ups become progressively refined, the theoretical and practical implications highlighted in this study provide a valuable framework for interpreting similar phenomena. As such, the research trajectory points towards enhancing our understanding of not just the immediate characteristics post-neutron star mergers, but also their contributions to cosmic ray propagation, element synthesis, and broad relativistic physics. Future work will likely exploit advancements in temporal and spectral resolution to refine these models further, potentially applying them to newly discovered transients.